Add pygmo to requirements-dev

[timokau]

Description

Pulled out of #118, since it unbreaks travis and is also impacting other PRs (#119).

Forgotten in b031340. Hopefully fixes
the tests on travis, which currently fail because pygmo isn't getting
installed. The travis test script installs dependencies from
requirements-dev.txt.

Motivation and Context

Checks on travis are broken due to a missing dependency.

How Has This Been Tested?

Fixed travis on #118.

Does this close/impact existing issues?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #120 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #120   +/-   ##
=======================================
  Coverage   60.18%   60.18%           
=======================================
  Files         116      116           
  Lines        7656     7656           
=======================================
  Hits         4608     4608           
  Misses       3048     3048