travis.yml: Use correct filenames for arm (v6/7) release files

In January, #2102 was closed with 5ae9d8b to build ARMv6/7 binaries.
This seems to be working, but there are no release files uploaded to
Github for this architecture, because the wrong filenames are used in
.travis-ci. This PR fixes that.

It must be noted that the dep install script is already modified to
expect a ARMv6/7 binary in the Github releases, so it errors out on
404 instead of emitting a clear error message which it did before:

```
Fetching https://github.com/golang/dep/releases/download/v0.5.1/dep-linux-arm
Request failed with code 404
```

Originally submitted by Github user confiks as #2135.

.travis.yml

@@ -106,8 +106,8 @@ jobs:
             - release/dep-linux-ppc64le.sha256
             - release/dep-linux-s390x
             - release/dep-linux-s390x.sha256
-            - release/dep-linux-armv6
-            - release/dep-linux-armv6.sha256
+            - release/dep-linux-arm
+            - release/dep-linux-arm.sha256
             - release/dep-linux-arm64
             - release/dep-linux-arm64.sha256
           skip_cleanup: true