Change Gemfile’s double quotation marks in plugin generator

Since the generated application’s Gemfile is using single quotation marks, it
seems fair to use it on plugin’s Gemfile as well.

Keep method's description/documentation consistent [ci skip]

Closes #15024.

These operations happen before instances are created.
The current behavior is misleading and can result in broken behavior.

Fix broken proc syntax for 1.9.3

This lets us avoid

1. A slow call to Hash#slice
2. An is_a? test
3. Extra hash allocations (from slice)
4. String allocations

etc.

avoids extra hash allocations on each call

this is due to:

  https://bugs.ruby-lang.org/issues/5994

Documentation, add examples of using an Enum scope [ci skip]

When scanning the Enum API page I found it easy to miss that scopes are generated.  Adding an example clarifies use and has more... *pop*...

Only need MINOR version level to test Ruby 2.1.x on Travis

cc http://blog.travis-ci.com/2014-04-28-upcoming-build-environment-updates/

Reverts dirty track restore on transactions rollback

do not test internals

We are reverting these commits, because there are other caveats related
to dirty attributes not being restored when a transaction is rollback.
For instance, nested transactions cannot proper restore the dirty
attributes state after a rollback.

At the moment, the dirty attributes are scoped by the transaction.
When we call `.save` on a record, the dirty attributes will be reset even
if the transaction gets rollback.

[related #13166]
[related #15018]
[related #15016]
[closes #15019]

This reverts commits
* bab48f0a
  * b0fa7cf3.
* 73fb39b6
  * 37c23892.
  * 8d8d4f15

Revert "Merge pull request #13166 from bogdan/transaction-magic"

Current docs are wrong. Does not accept strftime inputs.

Use ruby 2.1.2 on travis

Change 'a' to 'an' [ci skip]

[ci skip] Minor improvements on routing.md.