Update unit_definitions.py

Change bohr unit to value from CODATA (http://physics.nist.gov/cgi-bin/cuu/Value?bohrrada0)

wrappers/python/simtk/unit/unit_definitions.py

@@ -79,7 +79,7 @@ yard = yards = Unit({yard_base_unit: 1.0})
 furlongs = furlong = yard.create_unit(scale=220.0, name="furlong", symbol="furlong")
 miles = mile = furlong.create_unit(scale=8.0, name="mile", symbol="mi")
 
-bohrs = bohr = angstrom.create_unit(scale=0.52917720859, name='bohr', symbol='r_0')
+bohrs = bohr = angstrom.create_unit(scale=0.52917721067, name='bohr', symbol='r_0')
 
 ############
 ### MASS ###