Quote text PRAGMAs in repo.cpp setTextPragma

When attempting to set the encoding PRAGMA, I got this error:

```
$ ./hphp/hhvm/hhvm test.php
Failed to initialize central HHBC repository:
  Failed to initialize connection to /home/makerpm/.hhvm.hhbc:
  RepoStmt::prepare(repo=0x7fe0fdb73a00) error: 'PRAGMA main.encoding =
  UTF-8;' --> (1) near "-": syntax error
```

This fixes it by quoting the PRAGMA value.

NOTE: that none of the *Pragma methods seem to be tested, nor
RepoQuery in general.