build: prevent eol modification in vendored code

When 'dep ensure -vendor-only' is run, dep will update the dependencies
in the vendor directory to match what's referenced in Gopkg.lock. It
does this without performing EOL modification.

Even if there are no changes to the dependencies, this shows up as
pending changes in tree.

```
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   vendor/github.com/Knetic/govaluate/LICENSE
	modified:   vendor/github.com/konsorten/go-windows-terminal-sequences/LICENSE
```

When you run `git diff` to see the changes, warnings like the following are
issued:

```
warning: CRLF will be replaced by LF in
vendor/github.com/Knetic/govaluate/LICENSE.
The file will have its original line endings in your working directory.
```

This is telling us that when we add the changes to the index, git will
convert the CRLF to LF leaving is with no changes in the index and
nothing to commit.

We can prevent this confusion by disabling the automatic text conversion
for files in the vendor folder.

This change disables text conversion for vendor/** and re-adds the files
with CRLF from our vendor tree.

Signed-off-by: Matthew Sykes <[email protected]>

.gitattributes

@@ -16,3 +16,4 @@
 *.yml text eol=lf
 *.zip binary
 LICENSE text eol=lf
+vendor/** -text