Pipfile: Use Ansible 2.5.1

I had been using this from the stable-2.5 branch for a few weeks in
order to work around some issues with Jinja2 and ansible-vault, but
now that version 2.5.1 has been released I can use it directly from
pip.

Pipfile

@@ -8,6 +8,7 @@ name = "pypi"
 PyYAML = "*"
 paramiko = "*"
 cryptography = "*"
+ansible = "==2.5.1"
 
 [dev-packages]
 

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "a30f260039fc4abb38a6d2c5f81dd97d55c754ff582d804e636e929abe5162d1"
+            "sha256": "15cee6a880bc7ef20671d60af10232ca38c022010724ce86c64da1fee36c4eb8"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -16,6 +16,13 @@
         ]
     },
     "default": {
+        "ansible": {
+            "hashes": [
+                "sha256:51f0c9411344741fb4f6dd8757e0fb6a52c4dcf53a1bfbbfee413e067e65001a"
+            ],
+            "index": "pypi",
+            "version": "==2.5.1"
+        },
         "asn1crypto": {
             "hashes": [
                 "sha256:2f1adbb7546ed199e3c90ef23ec95c5cf3585bac7d11fb7eb562a3fe89c64e87",
@@ -87,6 +94,7 @@
                 "sha256:edabd457cd23a02965166026fd9bfd196f4324fe6032e866d0f3bd0301cd486f",
                 "sha256:fdf1c1dc5bafc32bc5d08b054f94d659422b05aba244d6be4ddc1c72d9aa70fb"
             ],
+            "markers": "platform_python_implementation != 'PyPy'",
             "version": "==1.11.5"
         },
         "cryptography": {