Build fix for CentOS
Build fix for CentOS

* 5/6 wget cannot verify nodejs.org wildcard cert. Adding `--no-check-certificate` for those two **specifically**.
* If build directory nad subdirectory exists but does not contain a `.git subdirectory, treat it as new and do a fresh pull (after removing builddir/nada).

HeinrichHartmann/nad,maier/nad,HeinrichHartmann/nad,HeinrichHartmann/nad,maier/nad,hjhart/nad,maier/nad,circonus-labs/nad,keithf4/nad,esproul/nad,keithf4/nad,circonus-labs/nad,maier/nad,esproul/nad,circonus-labs/nad,keithf4/nad,circonus-labs/nad,keithf4/nad,esproul/nad,HeinrichHartmann/nad,esproul/nad,HeinrichHartmann/nad,hjhart/nad,maier/nad,HeinrichHartmann/nad,keithf4/nad,esproul/nad,hjhart/nad,hjhart/nad,maier/nad,circonus-labs/nad,hjhart/nad,circonus-labs/nad