Fix Linux UCS2 using 2.7.13 with UCS4 instead of UCS2 sometimes

There were two versions of 2.7.13 installed on the system, so Pants would sometimes choose an unintended version and would be inconsistent.

build-support/docker/travis_ci_py27_ucs2/Dockerfile

@@ -1,13 +1,17 @@
 # Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-# This file duplicates travis_ci/Dockerfile, except it installs Python 2.7 instaed of Python 3.6
+# This file duplicates travis_ci/Dockerfile, except it installs Python 2.7 instead of Python 3.6
 # and adds the env var PYTHON_CONFIGURE_OPTS to install it with UCS2.
 
 # Use our custom Centos6 image for binary compatibility with old linux distros.
 FROM pantsbuild/centos6:latest
 
-ARG PYTHON_2_VERSION=2.7.13
+# Note we use 2.7.15, rather than 2.7.13, as the centos6 image already comes with 2.7.13
+# installed, which uses UCS4 instead of UCS2. By using 2.7.15, `ci.sh` will set the interpreter
+# constraints to `CPython==2.7.15` when bootstrapping the PEX, which will disambiguate the
+# interpreter version and ensure we always use 2.7.15 with UCS2. 
+ARG PYTHON_2_VERSION=2.7.15
 RUN yum install sqlite-devel -y
 ENV PYENV_ROOT /pyenv-docker-build
 RUN mkdir ${PYENV_ROOT}