Add ./node_modules/.bin to $PATH

travis-ci/travis-cookbooks#786 removed it, but
this broke many Ember-based repositories, so we will reinstate it.

This is meant to be a hot fix, which would be reverted after a new set
of build images are rolled out.

lib/travis/build/script/node_js.rb

@@ -17,6 +17,7 @@ def export
 
         def setup
           super
+          prepend_path './node_modules/.bin'
           convert_legacy_nodejs_config
           update_nvm
           nvm_install
@@ -212,6 +213,12 @@ def install_yarn
             sh.echo   "Setting up \\$PATH", ansi: :green
             sh.export "PATH", "$HOME/.yarn/bin:$PATH"
           end
+
+          def prepend_path(path)
+            sh.if "$(echo :$PATH: | grep -v :#{path}:)" do
+              sh.export "PATH", "#{path}:$PATH", echo: true
+            end
+          end
       end
     end
   end