[2.3] Fixed the proxy-manager version constraint

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

##### This pull request fixes the `ocramius/proxy-manager` version constraint in the 2.3 branch.

`"~0.3.1"` is far cleaner than `">=0.3.1,<0.4-dev"`, and does the same thing.