Liquidsoap 1.3.1

This is a bugfix release, addressing a couple of bugs from the recent 1.3.0 release. Changelog is:

New:

• Allow any tags allowed in "encoder.encoder.export" settings in vorbis streams (#418)

• Allow "audio/mp3" mime-type for mp3 in file resolution protocol. (#451)

Fixed:

• Fixed run_process, get_process_lines, get_process_output when compiling with OCaml <= 4.03 (#437, #439)

• Calls to wait_for while the scheduler isn't running (#442)

• Revert default handling of environment in run_process, get_process_lines, get_process_output to passing calling process' environment by default.