Use prebuildify-cross

[vweevers]

Which has been rewritten, see prebuild/prebuildify-cross#7. It's a github dependency for now; waiting for npm ownership.

In addition, CentOS & Alpine prebuilds are now built together in one Travis job. Simplifies the matrix.

[vweevers]

All good.

$ file prebuilds/*/*
prebuilds/linux-x64/node.napi.glibc.node: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=85491e89fa227f9012749aee0a6c980438605e4c, stripped
prebuilds/linux-x64/node.napi.musl.node:  ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, stripped
prebuilds/darwin-x64/node.napi.node: Mach-O 64-bit bundle x86_64
prebuilds/android-arm64/node.napi.armv8.node: ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically linked, stripped
prebuilds/android-arm/node.napi.armv7.node:   ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, stripped
prebuilds/linux-arm64/node.napi.armv8.node:   ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically linked, stripped
prebuilds/linux-arm/node.napi.armv7.node:     ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, stripped