upgrade our pypy2 5.6 builders to 5.7.1 (#3558)

diff --git a/.travis.yml b/.travis.yml
index 1816266..a00b869 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -25,7 +25,7 @@
           env: TOXENV=py36
         - env: TOXENV=pypy-nocoverage PYPY_VERSION=2.6.1
         - env: TOXENV=pypy PYPY_VERSION=4.0.1
-        - env: TOXENV=pypy PYPY_VERSION=5.6.0
+        - env: TOXENV=pypy PYPY_VERSION=5.7.1
         - python: 2.7
           env: TOXENV=py27 OPENSSL=1.1.0e
         - python: 3.5
@@ -65,7 +65,7 @@
         - language: generic
           os: osx
           osx_image: xcode8.1
-          env: TOXENV=pypy-nocoverage CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1 PYPY_VERSION=5.6.0
+          env: TOXENV=pypy-nocoverage CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1 PYPY_VERSION=5.7.1
         - language: generic
           os: osx
           osx_image: xcode8.1