#10428: Added tests for config.assets.precompile

Tests for bug in sprocket-rails:
  do not use value of configuration options
  which changed after environment loaded
Fix documents for create_join_table
Saying gemcutter => rubygems [ci skip]
Merge pull request #11043 from arunagw/gemcutter_to_rubygems

Saying gemcutter => rubygems [ci skip]
Merge pull request #11040 from satococoa/fix-create_join_table-document

Fix documents for create_join_table [ci skip]
Merge pull request #10537 from jetthoughts/10428_bug_with_config_assets_precompile

 #10428: Added tests for config.assets.precompile