Use the old default read size for python < 2.7.12

Also install 2.7.12 on travis.

scripts/install.sh

@@ -85,7 +85,7 @@ for var in "$@"; do
       install 2.7.8 python2.7.8
       ;;
     2.7)
-      install 2.7.11 python2.7
+      install 2.7.12 python2.7
       ;;
     3.2)
       install 3.2.6 python3.2

src/gevent/_sslgte279.py

@@ -164,6 +164,9 @@ _create_default_https_context = create_default_context
 # Backwards compatibility alias, even though it's not a public name.
 _create_stdlib_context = _create_unverified_context
 
+import sys
+# This changed with the fix for python bug #23804
+_DEFAULT_READ_LEN = 0 if sys.version_info[:3] < (2, 7, 12) else 1024
 
 class SSLSocket(socket):
     """
@@ -296,7 +299,7 @@ class SSLSocket(socket):
             # EAGAIN.
             self.getpeername()
 
-    def read(self, len=1024, buffer=None):
+    def read(self, len=_DEFAULT_READ_LEN, buffer=None):
         """Read up to LEN bytes and return them.
         Return zero-length string on EOF."""
         self._checkClosed()