remove pytest requirement & make languagecheck an optional dependency

.travis.yml

@@ -4,6 +4,7 @@ before_install:
   - python --version
   - pip install -U pip
   - pip install -U pytest
+  - pip install git+https://github.com/jxmorris12/language-check
 install:
   - pip install . # ".[test]" . # install package + test dependencies
 script: pytest # run tests

requirements.txt

@@ -1,6 +1,5 @@
 editdistance
 filelock
-language-check
 lru-dict
 nltk
 numpy
@@ -16,4 +15,3 @@ tensorflow_hub
 terminaltables
 tqdm
 visdom
-pytest