MAINT: Peg pip version and fix incompatibility with pip 7.0.3

In d01e718373, we made a change for
compatibility with a change introduced in pip 7.0.0. Then the pip
maintainers went and reverted the change in pip 7.0.2, thereby
breaking our requirements file for the second time in ten days. This
change reverts the prior change so that we are once again compatible
with the current version of pip.

Also, to make it easier for people to avoid pip compatibility issues,
we're now specifying pip>=7.0.3 in etc/requirements.txt.

etc/requirements.txt

@@ -1,3 +1,6 @@
+# Incompatible with earlier PIP versions
+pip>=7.0.3
+
 # Logging
 Logbook==0.9.1
 

etc/requirements_dev.txt

@@ -23,7 +23,7 @@ mistune==0.5.1
 # Example scripts that are run during unit tests use the following:
 
 # Required by tornado
-"backports.ssl-match-hostname==3.4.0.2;python_version<'3.0'"
+backports.ssl-match-hostname==3.4.0.2;python_version<'3.0'
 certifi==2015.4.28
 
 # tornado and pyparsing are required by matplotlib