Add a NEWS section
Update a note on required versions of Ruby
Stop running tests with MRI 1.9.3 on Travis CI
Run tests also with MRI 2.1.0 on Travis CI
Update the Philosophy on BranchVersion
Add the introduction on Branches
Add scenarios about installing with branches
Add scenarios about upgrading with branches
Add scenarios about changing tracking branches
Add scenarios about detecting incompatible branch
Define "$repo has $branch" step
Allow using digits as variable names
Enable to refer the revision of each veresion
Add PlainVersion, as a copy of Version
Change Version as a proxy of PlainVersion
Add BranchVersion
Support BranchVersion in Version.create
Refine VersionConstraint specs a bit
Add missing specs on VersionConstraint.parse
Ignore trailing spaces in a version constraint