Bump pydrake_common_install_test to medium

Our linux-xenial-clang-bazel-*-debug builds are sometimes hitting the 60s
timeout in CI, possibly due to I/O contention.  In any case, running and
validating a full install seems like a fair test to be marked not-small.

drake/bindings/BUILD.bazel

@@ -155,7 +155,7 @@ drake_py_test(
 # `no_everything` because `libgurobi70.so` is not found [Issue #7283].
 drake_py_test(
     name = "pydrake_common_install_test",
-    size = "small",
+    size = "medium",
     srcs = ["python/pydrake/test/testCommonInstall.py"],
     data = ["//:install"],
     main = "python/pydrake/test/testCommonInstall.py",