Fix issue #2 that trackTintColor was not drawn on arm64 architecture.

This is the same fix as the original DACircularProgress had:
https://github.com/danielamitay/DACircularProgress/commit/6d06b42f9ba13a72c49f959fa07cc15e6a53f1c2
https://github.com/danielamitay/DACircularProgress/issues/17
Fixed a compiler warning that a CGFloat type did not match on arm64 architecture.
Updated the podspec version number to 0.2.1 for the fix of issue #2.
Merge branch 'support-arm64'