Catch YAML::SyntaxError, not just SyntaxError.

In new versions of Psych, YAML::SyntaxError (which is
Psych::SyntaxError) is no longer a subclass of ::SyntaxError.
Add Psych 2.0.0 to the gemspec.
Merge pull request #1 from PagerDuty/catch_yaml_syntaxerror

Catch YAML::SyntaxError, not just SyntaxError.