latest python version (3.6.4) (#496)

Update buildpack to latest version of Python. 

bin/compile

@@ -34,8 +34,8 @@ ENV_DIR=$3
 export BUILD_DIR CACHE_DIR ENV_DIR
 
 # Python defaults
-DEFAULT_PYTHON_VERSION="python-3.6.3"
-LATEST_3="python-3.6.3"
+DEFAULT_PYTHON_VERSION="python-3.6.4"
+LATEST_3="python-3.6.4"
 LATEST_2="python-2.7.14"
 
 DEFAULT_PYTHON_STACK="cedar-14"

test/run

@@ -12,7 +12,7 @@ testPipenvLock() {
 
 testPipenvVersion() {
   compile "pipenv-version"
-  assertCaptured "3.6.3"
+  assertCaptured "3.6.4"
   assertCapturedSuccess
 }
 
@@ -78,7 +78,7 @@ testPython2() {
 
 testPython3() {
   compile "python3"
-  assertCaptured "python-3.6.3"
+  assertCaptured "python-3.6.4"
   assertCapturedSuccess
 }