More bazel --incompatible fixes (#161)

[mfarrugi]

This gets us from

bazel-20 test //... --all_incompatible_changes --incompatible_disable_deprecated_attr_params=false --incompatible_depset_is_not_iterable=false --incompatible_depset_union=false --incompatible_new_actions_api=false

to

bazel-20 test  --all_incompatible_changes //... @examples//... --incompatible_disable_deprecated_attr_params=false --incompatible_new_actions_api=false

Remaining issues:

>> bazel-20 test  --all_incompatible_changes //... @examples//... --keep_going

INFO: Invocation ID: deecf474-694f-4adc-9995-b09b2a5fc850
DEBUG: /home/marco/.cache/bazel/_bazel_marco/4e38d4394f29c071904de81d6fb3e103/external/bazel_skylib/skylark_library.bzl:23:1: WARNING: skylark_library.bzl is deprecated and will go away in the future, please use bzl_library.bzl instead.                  
ERROR: /home/marco/.cache/bazel/_bazel_marco/4e38d4394f29c071904de81d6fb3e103/external/bazel_tools/tools/build_rules/test_rules.bzl:308:17: Traceback (most recent call last):                                                                                
        File "/home/marco/.cache/bazel/_bazel_marco/4e38d4394f29c071904de81d6fb3e103/external/bazel_tools/tools/build_rules/test_rules.bzl", line 306                                                                                                         
                rule(attrs = {"file": attr.label(mand...))}, <3 more arguments>)
        File "/home/marco/.cache/bazel/_bazel_marco/4e38d4394f29c071904de81d6fb3e103/external/bazel_tools/tools/build_rules/test_rules.bzl", line 308, in rule                                                                                                
                attr.label(mandatory = True, allow_files = Tr..., ...)
'single_file' is no longer supported. use allow_single_file instead. You can use --incompatible_disable_deprecated_attr_params to temporarily disable this check.                                                                                             
ERROR: error loading package 'test': Extension file 'tools/build_rules/test_rules.bzl' has errors

Does it make sense that there are errors in @bazel_tools?

[mfarrugi]

This was a separate breakage

[mfarrugi]

Not clear why this needed --incompatible_depset_is_not_iterable=false to work, but I refactored it out.

[laurentlb]

Looks good, thanks!