Update to Singularity/3.5.2-GCC-5.4.0-2.26, fix --use flag

• Update Singularity version to Singularity/3.5.2-GCC-5.4.0-2.26, as previous (Singularity/2.6.1-GCC-5.4.0-2.26) was erroring out on singularity: "docker://ewels/multiqc:dev"

• Also improved syntax message in ./install.sh

• Also fixed a logic problem with --use flag (adds --use-conda and/or --use-singularity, if the directives are present in Snakefile)