change depends to 3.0.0

[javierluraschi]

Tried basic tibble functionality with 3.1.0 and could not spot any issues. Also tried with 3.0.0 but it depends on lazyeval which requires 3.1.0 since it depends on this addition wch/r-source@2e5fdef and therefore, supporting 3.0.0 would require additional work in lazyeval.

NOTE: Dependent on hadley/lazyeval#87

Fixes #189.

[codecov-io]

Current coverage is 99.59% (diff: 100%)

Merging #187 into master will not change coverage

@@             master       #187   diff @@
==========================================
  Files            16         16          
  Lines           738        738          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits            735        735          
  Misses            3          3          
  Partials          0          0          

Powered by Codecov. Last update 7c4a712...f73ab42

[hadley]

Could you please also add the appropriate tests to travis to ensure that we don't accidentally regress?

I'm happy to enforce the principle that tidyverse packages should work with R 3.0

[krlmlr]

• AppVeyor is a good way to test R versions prior to oldrel (currently 3.2.5).
• The new backports package provides compatibility wrappers for supporting R >= 3.0.0 without having to worry too much about interface changes in the base functions.

[krlmlr]

Closing in favor of #209.