Unpin GSD from version 1.7.0

[IAlibay]

Reverts #2320

Changes made in this Pull Request:

• As per build 1.8.0 showing up for python 2.7 on conda glotzerlab/gsd#30 the py2.7 packages for GSD 1.8.0 have now been labelled as broken on conda-forge and shouldn't be picked up by travis-ci anymore.
• I have therefore unpinned the version from 1.7.0.
• However, since the max version for py2.7 is now 1.7.0, I haven't changed setup.py, I can revert that too if the consensus is that it isn't necessary.

PR Checklist

• Tests? - Not needed
• Docs? - Not needed
• CHANGELOG updated? - Not needed
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #2321 into develop will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop    #2321     +/-   ##
==========================================
+ Coverage    89.64%   89.75%   +0.1%     
==========================================
  Files          157      173     +16     
  Lines        19547    21536   +1989     
  Branches      2804     2804             
==========================================
+ Hits         17523    19329   +1806     
- Misses        1431     1615    +184     
+ Partials       593      592      -1

Impacted Files	Coverage Δ
core/util.py	100% <0%> (ø)
transformations/__init__.py	100% <0%> (ø)
auxiliary/base.py	89% <0%> (ø)
util.py	88.15% <0%> (ø)
topology/base.py	97.67% <0%> (ø)
__init__.py	91.89% <0%> (ø)
topology/__init__.py	100% <0%> (ø)
visualization/__init__.py	100% <0%> (ø)
coordinates/__init__.py	100% <0%> (ø)
datafiles.py	26.66% <0%> (ø)
... and 7 more

---

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[orbeckst]

Can you try a clean git revert of the other commit (i.e. also revert setup.py) and see if the tests still pass? The less extra code the better.

Many thanks for following up with upstream!

[IAlibay]

Thanks, it should now all be reverted in 5c0d166 @orbeckst