Fix chirp mass range for 150914 to correct values for *detector frame*.

Update the uniform prior limits on the chirp mass in the GW150914.prior prior file to reflect the detector frame chirp mass (from GW150914-properties paper, the 90% interval on detector frame chirp mass is 28.5 - 32.5 MSun; see Table 1 of Abbott, et al. (2016).

bilby/gw/prior_files/GW150914.prior

 mass_ratio = bilby.gw.prior.UniformInComponentsMassRatio(name='mass_ratio', minimum=0.125, maximum=1)
-chirp_mass = bilby.gw.prior.UniformInComponentsChirpMass(name='chirp_mass', minimum=25, maximum=31)
+chirp_mass = bilby.gw.prior.UniformInComponentsChirpMass(name='chirp_mass', minimum=25, maximum=35)
 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)