Do not treat warnings as errors

Appveyor build fails treating warnings as errors. The warning is that
License is non-standard. The license issue was debated when the
package was created for @ropensci:

ropensci/software-review#118 (comment)

Message:
> checking DESCRIPTION meta-information ... WARNING
> Non-standard license specification:
>   CC0-1.0 + file LICENSE

https://ci.appveyor.com/project/timtrice/rrricanesdata-on6xt/builds/21296022/job/n3iygk4wgn1stu00#L276

Source:
https://github.com/ropensci/rrricanesdata/blob/master/appveyor.yml#L16

Fix:
The issue lies in the WARNINGS_ARE_ERRORS. Per the issue below (and the
latest README for kmrlmlr/r-appveyor), set this to FALSE by leaving
the field empty.

krlmlr/r-appveyor#79
https://github.com/krlmlr/r-appveyor/blob/ac6254e6d2e8629e037a0ddcfde985661487cd0a/README.md

appveyor.yml

@@ -14,7 +14,7 @@ install:
 
 environment:
   global:
-    WARNINGS_ARE_ERRORS: false
+    WARNINGS_ARE_ERRORS:
   matrix:
   - R_VERSION: 3.3.3
     R_ARCH: x64