Cannot GET /

Parece que o caminho utilizado no browsersync não existe. Compartilhe seu gruntfile.js para que possamos dar uma olhada.

Lembrando que o gruntfile.js deve estar dentro da pasta do projeto e não dentro de src.

module.exports = function (grunt) {
    grunt.initConfig({
        copy:{
            public:{
                expand: true,
                cwd: 'public',
                src: "**",
                dest: 'dist'
            }
        },

        clean:{
            dist:{
                src: 'dist'
            }
        },

        useminPrepare:{
            html: 'dist/**/*.html'
        },

        usemin:{
            html: 'dist/**/*.html'
        },

        imagemin:{
            public:{
                expand: true,
                cwd: 'dist/images',
                src: "**/*.{png,jpg,gif}",
                dest: 'dist/images'
            }
        },

        rev:{
            options:{
                encodinf: 'utf8',
                algorithm: 'md5',
                length: 8
            },
            imagens:{
                src: "dist/images/**/*.{png,jpg,gif}"
            },
            minificados:{
                src: ["dist/js/**/*.min.js","dist/css/**/*.min.css"]
            }
        },

        coffee:{
            compilar:{
                expand: true,
                cwd: 'public/coffee',
                src: "**/*.coffee",
                dest: 'public/js',
                ext: '.js'
            }
        },

        less:{
            compilar:{
                expand: true,
                cwd: 'public/less',
                src: "**/*.less",
                dest: 'public/css',
                ext: '.css'
            }
        },

        watch:{
            coffee:{
                options:{
                    event: ['added','changed']
                },
                files: 'public/coffee/**/*.coffee',
                tasks: 'coffee:compilar'
            },
            less:{
                options:{
                    event: ['added','changed']
                },
                files: 'public/less/**/*.less',
                tasks: 'less:compilar'
            },
            js:{
                options:{
                    event: ['added','changed']
                },
                files: 'public/js/**/*.js',
                tasks: 'jshint:js'
            }
        },

        jshint:{
            js:{
                src: ['public/js/**/*.js']
            }
        },

        browserSync:{
            public:{
                bsFiles:{
                    watchTask: true,
                    src: ['public/**/*']
                },
                option:{
                    server:{
                        baseDir: 'public'
                    }
                }
            }
        }
    });

    grunt.registerTask('server', ['browserSync','watch']);    
    grunt.registerTask('dist', ['clean','copy']);    
    grunt.registerTask('minifica', ['useminPrepare','concat','uglify','cssmin','rev','usemin','imagemin']);

    grunt.registerTask('default', ['dist','minifica']);

    grunt.loadNpmTasks('grunt-contrib-copy');
    grunt.loadNpmTasks('grunt-contrib-clean');
    grunt.loadNpmTasks('grunt-contrib-concat');
    grunt.loadNpmTasks('grunt-contrib-uglify');
    grunt.loadNpmTasks('grunt-contrib-cssmin');
    grunt.loadNpmTasks('grunt-contrib-imagemin');
    grunt.loadNpmTasks('grunt-contrib-less');
    grunt.loadNpmTasks('grunt-contrib-coffee');
    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.loadNpmTasks('grunt-contrib-jshint');

    grunt.loadNpmTasks('grunt-rev');
    grunt.loadNpmTasks('grunt-browser-sync');
    grunt.loadNpmTasks('grunt-usemin');
}

Dentro de public há algum arquivo chamado index.html?