Resolve issue with nightly CI breaking

In !573, the GW150914 prior was removed from the default directory. The
tests rely on this in order to run, this adds it back in.

bilby/gw/prior_files/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')
+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')
+a_2 = Uniform(name='a_2', minimum=0, maximum=0.99, boundary='reflective')
+tilt_1 = Sine(name='tilt_1', boundary='reflective')
+tilt_2 = Sine(name='tilt_2', boundary='reflective')
+phi_12 = Uniform(name='phi_12', minimum=0, maximum=2 * np.pi, boundary='periodic')
+phi_jl = Uniform(name='phi_jl', minimum=0, maximum=2 * np.pi, boundary='periodic')
+luminosity_distance = PowerLaw(alpha=2, name='luminosity_distance', minimum=50, maximum=2000, unit='Mpc', latex_label='$d_L$')
+dec =  Cosine(name='dec', boundary='reflective')
+ra =  Uniform(name='ra', minimum=0, maximum=2 * np.pi, boundary='periodic')
+theta_jn =  Sine(name='theta_jn', boundary='reflective')
+psi =  Uniform(name='psi', minimum=0, maximum=np.pi, boundary='periodic')
+phase =  Uniform(name='phase', minimum=0, maximum=2 * np.pi, boundary='periodic')