Cap ruamel.yaml version because of breaking change in 0.15.52 (#865)

Author: tetron

requirements.txt

@@ -1,5 +1,5 @@
 requests>=2.4.3
-ruamel.yaml>=0.12.4,<0.16
+ruamel.yaml>=0.12.4,<=0.15.51
 rdflib>=4.2.2,<4.3
 shellescape>=3.4.1,<3.5
 schema-salad>=2.7.20180719125426,<3

setup.py

@@ -51,7 +51,7 @@
       install_requires=[
           'setuptools',
           'requests >= 2.4.3',
-          'ruamel.yaml >= 0.12.4, < 0.16',
+          'ruamel.yaml >= 0.12.4, <= 0.15.51',
           'rdflib >= 4.2.2, < 4.3.0',
           'shellescape >= 3.4.1, < 3.5',
           'schema-salad >= 2.7.20180719125426, < 3',