Release 1.0.8

[WardBrian]

This issue is to track changes since 1.0.7 and propose a new release. Things which have changed:

• install_cmdstan now downloads the correct CmdStan for non-x86 Linux machines (#616).
• Improved reporting of errors during compile() (#620).
• Fixed some edge cases in mixing arguments of the optimize() function (#619).
• Fixed how NaN and infinite numbers were serialized to JSON (#623).
• Removed dependency on ujson. For now, all JSON serialization is done with the Python standard library.
• Added a timeout parameter to all model methods which can be used to terminate the CmdStan process after the specified time (#621).

If we'd like to do a release soon, I think it should still be 1.0.8. If we decide to wait until the next version of CmdStan and we address #592 and #593, it should be 1.1.0