Otherwise build breaks with boost>=1.70

Should fix #11

Ignore type_error while parsing timestamp of read receipts, because
Synapse sends out data for them, when using workers.

That's what the README suggests and - more importantly - the cmake
config file works with.
Also use angle brackets instead of quotation marks in two cases.

This will cause the http::Client to not block & exit immediately
when we're calling Client::close

- Remove the one time key that is used to create an inbound OlmSession.

Add a temporary fix for travis-ci with regard to wrongly exported include directories.