Make this script 1.8.7/1.9.3 compatible.

The current directory was removed from the Ruby load path in 1.9.
Checkout the branch of the tag, not the tag itself.

By convention, the VERSION file will contain a branch such as master or anand or a tag such as anand-1.

When the build.rb is run:
For master branch, we want to checkout master and reset to the sha1 of master.
For anand branch, we want to checkout anand and reset to the sha1 of anand.
For anand-1 tag, we want to checkout anand branch and reset to the commit that anand-1 references.
rake build:upload learned to upload VERSION file.

build.rb now depends on having the VERSION file locally via Build::GitCheckout.
Note: existing scripts expecting to run ruby ~/manageiq/build.rb need to be updated to use the new location: ruby ~/manageiq/build/build.rb