Merge branch 'gw150914_example_prior_fix' into 'master'

adjust prior to prevent chirp_mass railing

Closes #426

See merge request lscsoft/bilby!635

examples/gw_examples/data_examples/GW150914.prior

 mass_ratio = Uniform(name='mass_ratio', minimum=0.125, maximum=1, boundary='reflective')
-chirp_mass = Uniform(name='chirp_mass', minimum=25, maximum=31, unit='$M_{\odot}$', boundary='reflective')
+chirp_mass = Uniform(name='chirp_mass', minimum=25, maximum=35, unit='$M_{\odot}$', boundary='reflective')
 mass_1 = Constraint(name='mass_1', minimum=10, maximum=80)
 mass_2 = Constraint(name='mass_2', minimum=10, maximum=80)
 a_1 = Uniform(name='a_1', minimum=0, maximum=0.99, boundary='reflective')