remove pandas pre-0.20.0 compat

[asfimport]

xref to ARROW-879

Reporter: Jeff Reback / @jreback
Assignee: Jeff Reback / @jreback

_{Note: This issue was originally created as ARROW-956. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 649
#649