travis fixups

make travis execute py3.7 again

modernise config

.travis.yml

@@ -15,6 +15,8 @@ addons:
     # needed for GMPY
     - libgmp-dev
 
+os: linux
+
 python:
   - 2.6
   - 2.7
@@ -27,7 +29,6 @@ python:
 # workaround for 3.7 not available in default configuration
 # travis-ci/travis-ci#9815
 dist: trusty
-sudo: false
 
 env:
   - TACKPY=true
@@ -55,8 +56,6 @@ matrix:
 # workaround for 3.7 not available in default configuration
 # travis-ci/travis-ci#9815
     - python: 3.7
-      dist: xenial
-      sudo: true
       env: M2CRYPTO=true
     - python: 2.7
       env: PYCRYPTO=true
@@ -67,8 +66,6 @@ matrix:
     - python: 3.6
       env: PYCRYPTO=true
     - python: 3.7
-      dist: xenial
-      sudo: true
       env: PYCRYPTO=true
     - python: 2.7
       env: GMPY=true
@@ -79,8 +76,6 @@ matrix:
     - python: 3.6
       env: GMPY=true
     - python: 3.7
-      dist: xenial
-      sudo: true
       env: GMPY=true
     - python: 2.7
       env: M2CRYPTO=true PYCRYPTO=true GMPY=true
@@ -91,8 +86,6 @@ matrix:
     - python: 3.6
       env: M2CRYPTO=true PYCRYPTO=true GMPY=true
     - python: 3.7
-      dist: xenial
-      sudo: true
       env: M2CRYPTO=true PYCRYPTO=true GMPY=true CC_COV=true
 
 before_install:
@@ -163,8 +156,6 @@ script:
         done
       fi
 
-sudo: false
-
 after_success:
  - travis_retry coveralls
  - if [[ $CC_COV == 'true' ]]; then coverage xml; ./cc-test-reporter after-build --exit-code $TRAVIS_TEST_RESULT; fi