Fix Triangular bounds

[ricardoV94]

Small fix to Triangular dist logp and logcdf bounds.

Before this PR the following evaluations gave wrong results (all should evaluate to -inf):

pm.Triangular.dist(lower=0, upper=1, c=2.0).logp(1.9).eval()           
array(0.64185389)

pm.Triangular.dist(lower=0, upper=1, c=-0.5).logp(-0.3).eval()          
array(0.55004634)

pm.Triangular.dist(lower=1, upper=0, c=0.5).logp(0.7).eval()
array(inf)

pm.Triangular.dist(lower=1, upper=0, c=2).logcdf(2).eval()              
array(nan)

pm.Triangular.dist(lower=1, upper=0, c=0.5).logcdf(2).eval()            
array(0.)

Depending on what your PR does, here are a few things you might want to address in the description:

• what are the (breaking) changes that this PR makes?
• important background, or details about the implementation
• are the changes—especially new features—covered by tests and docstrings?
• linting/style checks have been run
• consider adding/updating relevant example notebooks
• right before it's ready to merge, mention the PR in the RELEASE-NOTES.md