Compiles with boost 1.67

[Fierthraix]

Fixes https://github.com/balint256/gr-baz/issues/55

Before boost 1.67 values passed to date_time structs would be auto-converted from floating point numbers to integers. Boost 1.67 disallows this to force user handling of special values (like NaN).

This converts the interval time back to an integer before being sent to posix_time::microseconds.

[webhat]

Is there a reason this isn't merged into the main branch?

[AdamVanScyoc]

I was about to submit a pull request with an equivalent fix

Should be merged, will also fix #60

@balint256 merge?