Use the ::Time constant to avoid colisions

Some other "progress bars" have their own idea of "Time" (see:
ruby-progressbar) and can break the expected usage in this Gem. So lets
be sure we're talking about the REAL ::Time.
Merge pull request #22 from stevenharman/resolve-real-Time-constant

Use the ::Time constant to avoid colisions
Add 2.1.1 to build list
Update README with a we're not dead yet
Use newer-style travis rbx specifier