It seems I had to upgrade to xenial to use cp37 on travis-ci
It seems I had to upgrade to xenial to use cp37 on travis-ci

xenial + python-3.7 ==> 200

```
% curl -I https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-3.7.tar.bz2
HTTP/2 200
x-guploader-uploadid: AEnB2UqRsEaLAUb0CD-r4E49I-SPtgZm0WTyy6x_og6kBIfcd_gMLWxNM1VSxMk6Lg0321kv3cg4vWi-aL8CjjEqUFqw1H3lxw
expires: Wed, 27 Mar 2019 02:28:05 GMT
date: Wed, 27 Mar 2019 01:28:05 GMT
cache-control: public, max-age=3600
last-modified: Thu, 07 Mar 2019 20:51:41 GMT
etag: "3e4b652e544030085a2f85a1a2e6edf3"
x-goog-generation: 1551991901703147
x-goog-metageneration: 1
x-goog-stored-content-encoding: identity
x-goog-stored-content-length: 65857925
content-type: application/x-bzip2
x-goog-hash: crc32c=FeNwXg==
x-goog-hash: md5=PktlLlRAMAhaL4Whoubt8w==
x-goog-storage-class: MULTI_REGIONAL
accept-ranges: bytes
content-length: 65857925
server: UploadServer
alt-svc: quic=":443"; ma=2592000; v="46,44,43,39"

```

trusty + python-3.7 ==> 404

```
% curl -I https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/14.04/x86_64/python-3.7.tar.bz2
HTTP/2 404
x-guploader-uploadid: AEnB2UoZcAwry0GJC6wq_pkekWtscWtOIQNAzldWDFJ3lyvD3FUp0ZSc1iv67btTKtSsqMWwiQcpB-oZw5IAp_xF_JaoZJNFRw
content-type: application/xml; charset=UTF-8
content-length: 244
date: Wed, 27 Mar 2019 01:28:10 GMT
expires: Wed, 27 Mar 2019 01:28:10 GMT
cache-control: private, max-age=0
server: UploadServer
alt-svc: quic=":443"; ma=2592000; v="46,44,43,39"

```

treasure-data/td-client-python