Change development versioning

No longer uses build timestamps by default when versioning because it

Leads to lots of gems when installing them in succession.
Instead development builds will simply have ".dev" appended.

Build timestamps can be turned on by passing :use_build => true

Also will now always increase version for development builds except on
"release" and "hotfix" branches.
Allow specifying use_build in versioning via ENV

Use rake USE_BUILD=1 or USE_BUILD=1 gem build drush-deploy.gemspec
to produce gem with build number.
Change log message when building gem in dirty wd
Merge branch 'feature/fix_build_version' into develop