Replace Ubuntu 16.04 with CentOS 7 for prebuilds

Using the new prebuildify-cross (prebuild/prebuildify-cross#7).

This makes the prebuilt binary for linux compatible with Debian 8,
Ubuntu 14.04, RHEL 7, CentOS 7 and other flavors with an old glibc.

Following Level/leveldown#674.

.travis.yml

@@ -10,7 +10,7 @@ jobs:
       env: [TEST=1]
     - os: linux
       node_js: node
-      env: [TEST=1, TEST_ELECTRON=1, BUILD_CMD=prebuild, BUILD_GROUP=linux-x64]
+      env: [TEST=1, TEST_ELECTRON=1, BUILD_CMD=prebuild-linux, BUILD_GROUP=linux-x64]
       addons:
         apt:
           packages:
@@ -35,13 +35,16 @@ before_install:
 script:
   - if [[ ! -z "$TEST" ]]; then npm run test; fi
   - if [[ ! -z "$TEST_ELECTRON" ]]; then npm run test-electron; fi
+  # Temporary to test prebuilds
+  - if [[ ! -z "$BUILD_CMD" ]]; then npm run $BUILD_CMD; fi
+  - if [[ ! -z "$BUILD_CMD" ]]; then file prebuilds/*/*; fi
 
 after_success:
   - if [[ ! -z "$TEST" ]]; then npm run coverage; fi
 
 before_deploy:
   - export ARCHIVE_NAME="${TRAVIS_TAG:-latest}-$BUILD_GROUP.tar.gz"
-  - npm run $BUILD_CMD -- --tag-libc
+  - npm run $BUILD_CMD
   - file prebuilds/*/*
   - tar -zcvf "$ARCHIVE_NAME" -C prebuilds .
 

package.json

@@ -13,6 +13,7 @@
     "coverage": "nyc report --reporter=text-lcov | coveralls",
     "rebuild": "npm run install --build-from-source",
     "prebuild": "prebuildify -t 8.14.0 --napi --strip",
+    "prebuild-linux": "prebuildify-cross -i linux",
     "download-prebuilds": "prebuildify-ci download",
     "hallmark": "hallmark --fix",
     "dependency-check": "dependency-check --no-dev -i napi-macros . test/*.js",
@@ -41,6 +42,7 @@
     "nyc": "^14.0.0",
     "prebuildify": "^3.0.0",
     "prebuildify-ci": "^1.0.4",
+    "prebuildify-cross": "github:prebuild/prebuildify-cross#rewrite",
     "readfiletree": "^1.0.0",
     "rimraf": "^3.0.0",
     "standard": "^14.0.0",