fix ruamel version

Author: mr-c

requirements.txt

@@ -1,5 +1,5 @@
 requests>=2.4.3
-ruamel.yaml>=0.12.4,<=0.15.66
+ruamel.yaml>=0.12.4,<=0.15.51
 rdflib>=4.2.2,<4.3
 shellescape>=3.4.1,<3.5
 schema-salad>=2.7.20180905124720,<3

setup.py

@@ -53,7 +53,7 @@
           'setuptools',
           'requests >= 2.6.1',  # >= 2.6.1 to workaround
           # https://github.com/ionrock/cachecontrol/issues/137
-          'ruamel.yaml >= 0.12.4, <= 0.15.66',
+          'ruamel.yaml >= 0.12.4, <= 0.15.51',
           'rdflib >= 4.2.2, < 4.3.0',
           'shellescape >= 3.4.1, < 3.5',
           'schema-salad >= 2.7.20180905124720, < 3',