Oracle Linux support

[crowesn]

We use Oracle Linux 8.7, which is derived from Redhat/Centos 8.

This gem includes a compatible binary, however the filename, I think constructed from os-release data, does not resolve to the packaged centos binary.

A dedicated binary for Oracle Linux is not present and the gem does not resolve to the compatible centos binary. We have a workaround with a symbolic link to the correct binary but it's brittle and I gather this could be corrected by adding an ol_8 option under wkhtmltopdf#L41

Shall I submit a patch?

Compatible binary: wkhtmltopdf_centos_8_amd64.gz
Constructed filename: wkhtmltopdf_ol_8.7_amd64.gz

$ cat /etc/os-release
NAME="Oracle Linux Server"
VERSION="8.7"
ID="ol"
ID_LIKE="fedora"
VARIANT="Server"
VARIANT_ID="server"
VERSION_ID="8.7"
PLATFORM_ID="platform:el8"
PRETTY_NAME="Oracle Linux Server 8.7"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:oracle:linux:8:7:server"
HOME_URL="https://linux.oracle.com/"
BUG_REPORT_URL="https://bugzilla.oracle.com/"

ORACLE_BUGZILLA_PRODUCT="Oracle Linux 8"
ORACLE_BUGZILLA_PRODUCT_VERSION=8.7
ORACLE_SUPPORT_PRODUCT="Oracle Linux"
ORACLE_SUPPORT_PRODUCT_VERSION=8.7

Related: #79