Update CHANGELOG entry for bridgedb-0.1.5.
Only read emails into logger if configured to log emails.

The variable name and logging text stating that the "email body" was
being logged was untrue ― the entire email text was logged, including
headers, which completely nullified all the other efforts to use
SAFE_LOGGING and keep client email addresses out of BridgeDB's log
files.

In commit 3a3fe60e159bfcb93f51cff861a0db794b867ad4, these lines were
changed to use `Util.logSafely(f.read())`. However, this is
inefficient. We shouldn't bother reading the entire email file if the
`Util.logSafely()` is only going to rewrite those contents with the
string `[scrubbed]`.

 * CHANGE the way the `bridgedb.EmailServer.composeEmail()` function
   logs email contents to use SAFE_LOGGING more efficiently.