Use different theme for PRs

[rosiel]

Purpose / why

Travis is breaking on PR's, as it can't use the github token to access the mkdocs-material-insiders theme. this is expected Travis behaviour for security reasons.

https://docs.travis-ci.com/user/pull-requests#pull-requests-and-security-restrictions

Switch to using normal Material for PRs.

Replace this text - Name some folks who may be interested, if documentation related mention @Islandora/documentation, or, if unsure, @Islandora/8-x-committers

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Checklist

Pull-request reviewer should ensure the following

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?
• Does this PR update the last updated on date on the documentation page?

Person merging should ensure the following

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?