FIX: update cryptography version to 2.5 or later

[jamesbeyond]

Summary of changes

Some cryptography dependency libraries not available anymore.
Update cryptography to version 1.5 or later for TravisCI installation to be passed

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@adbridge

[ciarmcom]

@jamesbeyond, thank you for your changes.
@adbridge @ARMmbed/mbed-os-maintainers please review.

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 10 of 10 test jobs passed
Build number : 1
Build artifacts