Update travis to use mkdocs insiders

[dannylamb]

Purpose / why

Enable premium fetaures of mkdocs-material

What changes were made?

Added an access token, and am using it to install premium mkdocs-material in travis

Verification

Still figuring that out, honestly. We need to use a premium feature to test.

Interested Parties

@Islandora/documentation

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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?

[dannylamb]

This is probably having issues reading the secret. I'll have to fiddle with this to make sure it's getting my access token.

[dannylamb]

Travis is happy with this now