Remove version constraints from setup.py

`pip` can behave unexpectedly with some version constraints.
  1. It will abort when constraints aren't satisfiable using the highest version
  2. It will not consider multiple constraints on a single dependency

In general, it is safer to remove constraints from setup.py, and allow
users to ensure install which-ever dependencies work.

constraints.txt

@@ -0,0 +1,4 @@
+cryptography>=1.5
+boto3>=1.1.1
+click>=6.6
+PyYAML>=3.10

setup.py

@@ -55,16 +55,16 @@ def read_markdown(*file_paths):
 
     setup_requires=[] + pytest_runner + setupext_pip,
     install_requires=[
-        'cryptography~=1.5',
-        'boto3>=1.1.1',
-        'click>=6.6',
+        'cryptography',
+        'boto3',
+        'click',
         'six'
     ],
     tests_require=[
         'pytest',
     ],
     extras_require={
-        'yaml': ['PyYAML>=3.10'],
+        'yaml': ['PyYAML'],
         'templates': ['jinja2'],
         'documentation': ['pyandoc'],
     },