Update dependency requirements (#457)

* Make foyer an optional dependency

* Split dependencies into required and required for development

* Update developer requirements

* Marked appropriate tests to check if Foyer is installed

* Remove duplicate line

* Remove openbabel and gsd from Appveyor testing

* Attempt to fix coverage dependency issue (#466)

* Attempt to fix coverage dependency issue

Currently, the most recent version of python-coveralls requires
`coverage==4.0.3`, while pytest-cov requires `Coverage>=4.4`.

The current fix seems to be to pin `pytest-cov` to a previous
version. This can be changed once:
z4r/python-coveralls#66

has been resolved.

* Hotfix for MDTraj MOL2 file issues

MDTraj has merged a fix for MOL2 file reading
mdtraj/mdtraj#1378

However, it has not been included in a new release on
`conda` yet.

Pinning to an older version without the MOL2 fixes
currently.

* Forgot to update Appveyor build script

* Bump to version 0.8.1

* Update changelog to 0.8.1

* Fixes issues with packmol input files (#474)

* Fixes issues with packmol input file

Also reports error based on process code instead of output,
which prevented report of error about input issues.

* Bump to version 0.8.1

* Update changelog to 0.8.1

* Small formatting nits

* Run `activate base` before `conda build`

See ContinuumIO/anaconda-issues#10211 (comment)

* Update changelog

* Re-pin mdtraj and pytest-cov

mosdef_gomc/utils/io.py

@@ -65,6 +65,18 @@ class DelayImportError(ImportError, SkipTest):
 
 MESSAGES['pybel'] = MESSAGES['openbabel']
 
+MESSAGES['foyer'] = '''
+The code at {filename}:{line_number} requires the "foyer" package
+
+foyer can be installed using:
+
+# conda install -c mosdef foyer
+
+or
+
+# pip install foyer
+'''
+
 
 def import_(module):
     """Import a module, and issue a nice message to stderr if the module isn't installed.
@@ -133,6 +145,13 @@ def import_(module):
 except ImportError:
     has_openbabel = False
 
+try:
+    import foyer
+    has_foyer = True
+    del foyer
+except ImportError:
+    has_foyer = False
+
 
 def get_fn(name):
     """Get the full path to one of the reference files shipped for utils.