Update cxserver to 31d6559

Changes:
* 31d6559 Use valid sample text for checking MT pair's status
*   17c523c Merge "MT: Introduce MTClient and all other MT clients inherit it"
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| * 9739d66 MT: Introduce MTClient and all other MT clients inherit it
* | d360b53 Fix indentation in package.json
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* 0edb820 Yandex machine translation backend client
* 71e0705 Improve mt error handling and reporting
*   fc5df07 Merge "Add hi-ur and ur-hi language pairs to config"
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| * d761911 Add hi-ur and ur-hi language pairs to config
* | 105a222 Fixed typo and spacing in mt/index.html

Change-Id: I3ef241a0d5bfba27175da5f46227f166f812b78b