fix named consts

[cristaloleg]

Found by go-critic linter via namedConst check.
From here: #960 (comment)

check-package: $GOPATH/src/github.com/google/go-github/github/activity_notifications_test.go:37:34: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/activity_notifications_test.go:38:34: namedConst: use time.March instead of 3
check-package: $GOPATH/src/github.com/google/go-github/github/activity_notifications_test.go:83:88: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/activity_notifications_test.go:101:108: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/gists_test.go:341:43: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/gists_test.go:532:41: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/gists_test.go:533:41: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/github_test.go:514:27: namedConst: use time.July instead of 7
check-package: $GOPATH/src/github.com/google/go-github/github/github_test.go:546:27: namedConst: use time.July instead of 7
check-package: $GOPATH/src/github.com/google/go-github/github/github_test.go:585:27: namedConst: use time.July instead of 7
check-package: $GOPATH/src/github.com/google/go-github/github/github_test.go:769:42: namedConst: use time.March instead of 3
check-package: $GOPATH/src/github.com/google/go-github/github/github_test.go:870:41: namedConst: use time.July instead of 7
check-package: $GOPATH/src/github.com/google/go-github/github/github_test.go:875:41: namedConst: use time.July instead of 7
check-package: $GOPATH/src/github.com/google/go-github/github/orgs_members_test.go:416:31: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/orgs_teams_test.go:631:31: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/repos_community_health_test.go:57:31: namedConst: use time.February instead of 2
check-package: $GOPATH/src/github.com/google/go-github/github/repos_stats_test.go:57:44: namedConst: use time.May instead of 5
check-package: $GOPATH/src/github.com/google/go-github/github/repos_stats_test.go:98:38: namedConst: use time.May instead of 5
check-package: $GOPATH/src/github.com/google/go-github/github/repos_stats_test.go:123:41: namedConst: use time.April instead of 4
check-package: $GOPATH/src/github.com/google/go-github/github/strings_test.go:57:30: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/strings_test.go:61:31: namedConst: use time.January instead of 1
check-package: $GOPATH/src/github.com/google/go-github/github/timestamp_test.go:23:34: namedConst: use time.January instead of 1

[gmlewis]

s/time.July/time.March/

I won't comment on any others... please make sure all the tests pass.
See the CONTRIBUTING.md file in the top of the repo for more details.

Thank you!

[cristaloleg]

Oh, such a silly mistake. Sorry for that :(

[gmlewis]

Very nice. Thank you, @cristaloleg.
Merging.