Set cachebust query instead of adding

[vcarl]

Resolve #169

[bartekn]

This looks good, but let me check if we still need to send random value with each request. This shouldn't be required if Horizon was sending cache-related headers.

[coveralls]

Coverage remained the same at 93.103% when pulling 64fa458 on vcarl:replace-c into 30f8be6 on stellar:master.

[ire-and-curses]

Stack Overflow suggests Cache-Control: no-store could resolve this without need for random strings at all (multiple confirming comments at https://stackoverflow.com/a/27514611/143804).

[bartekn]

Thanks!

We will eventually remove this as stellar/go#476 is a proper solution.