Updating regex used to split sphinx version string.

The latest stable sphinx version string is '1.2+'.
The regex pattern '[a-z]' fails to match the '+' character
leading to a `ValueError` being raised when it attempts to
cast '2+' to an int. Replacing '[a-z]' with '[^0-9]' resolves this.
Merge pull request #2703 from androomerrill/plot_directive_sphinxext_fix

Updating regex used to split sphinx version string.