strip newlines off the end of env variable values

When putting multiline expressions in the `env` section in the
.rock.yml, the values wind up with an extra newline on the end. This
strips that off.

For example:

.rock.yml:

    env:
      INTEGRATION_DB_HOST: |
        `case $JOB_NAME in
          *-qa)   echo -n "jenkins-test-db.qa.example.net"    ;;
          *-prod) echo -n "jenkins-test-db.prod.example.net"  ;;
          *)      echo -n "jenkins-test-db.dev.example.net"   ;;
        esac`

$ rock env

    # ---- SNIP ---
    export INTEGRATION_DB_HOST="`case $JOB_NAME in
      *-qa)   echo -n "jenkins-test-db.qa.example.net"    ;;
      *-prod) echo -n "jenkins-test-db.prod.example.net"  ;;
      *)      echo -n "jenkins-test-db.dev.example.net"   ;;
    esac`
    "

Note the newline after the `esac`'s closing backtick.