Move jshint configuration to .jshintrc

When provided with inline configuration in a gulp task jshint will
still try to load a configuration file from the current directory
or the user's home directory. If user has a global .jshintrc file
that sets different linting options this could lead to `npm test`
output being different from the CI one.

jshint only uses the first file it finds, and .jshintrc in current
directory or any parent of the current directory takes precedence
over the user one, so moving jshint configuration from gulpfile to
.jshintrc should make `npm test` produce the same outcome regardless
of the user config.

.jshintrc

@@ -0,0 +1 @@
+{"esversion": 6, "esnext": false}

gulpfile.babel.js

@@ -138,7 +138,7 @@ gulp.task('lint:sass', () => gulp
 
 gulp.task('lint:js', () => gulp
   .src(paths.src + 'javascripts/**/*.js')
-    .pipe(plugins.jshint({'esversion': 6, 'esnext': false}))
+    .pipe(plugins.jshint())
     .pipe(plugins.jshint.reporter(stylish))
     .pipe(plugins.jshint.reporter('fail'))
 );