Update podspec

Bump version number and update podspec

Leverage new APIs to efficiently activate/deactivate arrays of constr…

…aints

- Use the class methods on NSLayoutConstraint to activate and
deactivate arrays of constraints. These can be more efficient than
activating/deactivating constraints individually.
- Fix the preprocessor conditionals so that OS X 10.10 and higher can
use the new constraint activation/deactivation APIs when available

Update version number to v2.0.1

Update podspec for v2.0.0

Update README and podspec

Update README and podspec

Update .travis.yml