correct dependencies in Makefile

Makefile

-paper.pdf: bbl
+paper.pdf: paper.bbl paper.tex
 	pdflatex paper.tex
 	pdflatex paper.tex
 
-bbl: paper.tex
+paper.bbl: paper.tex paper.bib
 	pdflatex paper.tex
 	bibtex paper.aux
 
-paper.tex:
+paper.tex: paper.md
 	pandoc -s --filter pandoc-fignos --filter pandoc-citeproc --filter pandoc-crossref --natbib paper.md -o paper.tex --bibliography paper.bib --listings --template revtex.template