Fix release infrastructure failures discovered during .69 release

Summary:
During the release of .69, we (fortmarek and me) discovered a couple of bits that needed
some intervention.
- `sdks/.hermesversion` was gitignored, so we could not commit that.
- `scripts/bump-hermes-version.js` was not executable, so we had to chmod +x to
  make it runnable.

Here I'm fixing it.

Changelog:
[Internal] [Changed] - Fix release infrastructure failures discovered during .69 release

Reviewed By: cipolleschi

Differential Revision: D36003808

fbshipit-source-id: c4d82ed5e2c63988699035ac84b0e87ed8894540

.gitignore

@@ -115,7 +115,6 @@ package-lock.json
 /packages/rn-tester/NativeModuleExample/ScreenshotManagerSpec*
 
 # Additional SDKs
-/sdks/.hermesversion
 /sdks/hermes
 /sdks/download
 

scripts/bump-hermes-version.js

@@ -12,6 +12,7 @@
 
 /**
  * This script walks a releaser through bumping the Hermes version for a release.
+ * It needs be executed on a release branch.
  */
 const {exit} = require('shelljs');
 const yargs = require('yargs');