Use string literals when constructing config_vars

This fixes `.profile.d/config_vars: line 2: -Xss512k: command not found` error when given a .release file of:

```
root@ip-10-250-15-201:~/slugrunner# tar xfO /opt/deis/runtime/slugs/player-hawthorn-v6.tar.gz ./.release
---
config_vars:
  PATH: /app/.jdk/bin:/usr/local/bin:/usr/bin:/bin
  JAVA_OPTS: -Xmx384m -Xss512k -XX:+UseCompressedOops
  MAVEN_OPTS: -Xmx384m -Xss512k -XX:+UseCompressedOops 
addons:
  heroku-postgresql:dev
```